Parker O-Ring Handbook.pdf

6.4.2 Metal Non-Extrusion Rings
In most cases it will be impracticable and unnecessary
(under 1380 bar 20,000 psi) to design bolt and rivet seal
glands that are self-tightening, i.e., tending to reduce rather
than increase seal extrusion clearance as pressure increases.
Adequate gland volume should be allowed, in keeping with
assembly tolerances. Two designs that have been helpful in
alleviating extrusion in ultra high pressure applications are
shown in Figure 6-2.
6.4.3 Leather Back-Up Rings
Leather was a standard back-up ring material for many years.
Manufacturers of these rings developed special processing
methods and impregnations for different types of applications.
Standard sizes were established for use with all standard
O-rings. If there is any question concerning the suitability
of leather for the application, consult the supplier. Leather
back-up rings are manufactured as continuous rings and in
most cases must be stretched during installation. Less damage
will be incurred to the back-up rings if they are soaked in oil
before installation. After installation, a short exposure to heat
will shrink the leather rings back to size. Leather back-up
rings should never be cut to facilitate installation.
6.5 Parbak Compound Information
Some back-up ring materials tend to leave deposits in the
micro fi ne grooves of the surface on which they rub. An ultra
smooth, wax-like surface results. Because an O-ring may wipe
all lubrication from such a surface, reverse stroking is dry
and greatly reduces seal life. Parbaks of N1444-90 and other
rubber compounds solve this problem. They do not leave a
deposit on the metal surface, thus lubrication remains.
The standard compound for Parbaks is N1444-90. Careful
engineering and research has produced N1444-90 which
has the best combination of characteristics for the majority
of back-up ring installations — broad temperature range,
proper hardness, long sealing life, and resistance to a great
number of fl uids. It is resistant to nearly all hydraulic fl uids
except certain non-fl ammable types such as Skydrol. It is
also resistant to air and water.
Parker O-Ring Handbook
Functional tests have proven that millions of cycles can be
obtained with Parbaks, showing their tremendous superiority
over the older types of back-up rings.
In addition to N1444-90, Parker Seal has developed other
compounds for installations requiring special characteristics.
Present capabilities include service at continuous temperatures
as high as 204°C (400°F).
Additional assistance in specifi ng and using Parbak rings
is available upon request by calling your Parker Seal
representative.
6.6 Parker Parbak 8-Series Dimensions
Parbaks will stretch up to 50%, and are quickly and easily
installed. Advantages of the contour design are obtained regardless
of how Parbaks are installed. They may be installed
with the concave face in either direction, toward or away
from the O-ring.
Table 6-1 provides Parker Parbak 8-Series Dimensions.
Scarf Cut Spiral Parbak
Figure 6-1: Typical Back-Up Rings
Thin Metal Section Will Expand
Due to Pressure and Maintain
Metal-to-Metal Fit
7° to 10°
Soft Metal Anti-
Extrusion Wedge Ring
Figure 6-2: Clearance Control for High Pressure Seals
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
Back-Up Rings
6-3